JP2007-274820A published by the Japan Patent Office proposes a linear actuator provided with a cylindrical tube having a closed end, a cylindrical yoke retaining permanent magnets arranged along an axial direction on an outer peripheral surface, and a plurality of coils disposed on an inner surface of the tube. This linear actuator is designed to drive the yoke in the axial direction relative to the tube.
The linear actuator of JP2007-274820A is also provided with a rod which is disposed in a bottom portion of the tube and can be moved forward into and backward from the yoke, a bearing disposed at an extreme end of the rod in sliding contact with an inner surface of the yoke, and a bearing disposed on the inner surface of the yoke in sliding contact with an outer peripheral surface of the rod. With these two bearings serving to guide the moving yoke, the linear actuator is configured to extend and contract smoothly.
At a proximal end of the yoke, there is provided a cylindrical protective cover having a closed end, the protective cover covering an open end of the tube. On an inner surface of an open end portion of the protective cover, there is provided a dust seal which slides along an outer peripheral surface of the tube. This dust seal prevents such foreign matter as dust, dirt and drops of water from entering an internal space of the linear actuator. In the linear actuator of JP2007-274820A, the internal space thereof is sealed by the dust seal as mentioned above.